1. Field of the Invention
The present invention relates to an X-ray computerized tomography apparatus (hereinafter referred to as an X-ray CT apparatus) and, more particularly, to image reconstruction in, for example, a third generation X-ray CT apparatus using a helical scan scheme in which the projection is performed while the bed is moved.
2. Description of the Related Art
FIG. 1 is a schematic block diagram showing a processing circuit associated with image reconstruction processing in a conventional X-ray CT apparatus. Projection data or views (hereinafter referred to as views) from a data acquisition system (not shown) are supplied to a pre-processing circuit 2. The preprocessing circuit 2 performs a logarithmic conversion and various corrections on the views. A raw data memory 4 and a central processing unit (hereinafter referred to as a CPU) 6 are connected to the output terminal of the pre-processing circuit 2. An output from the preprocessing circuit 2 is stored, as a reconstructed image, in an image memory 12 through a convolution processing circuit (hereinafter referred to as a CONV circuit) 8 and a back projection processing circuit (hereinafter referred to as a BP circuit) 10. The reconstructed image data in the image memory 12 is supplied to contrast scale circuit 14 to be converted into a CT image data. The CT image data is stored in a CT image memory 14 and is supplied to a display device (not shown).
A CPU 16 is connected to the CONV circuit 8, the BP circuit 10 and the contrast scale circuit 14. In addition, a parameter setting device 18 for setting various parameters for back projection processing under the control of the CPU 16 is connected to the BP circuit 13. Although not shown, various buffers are included in the CONV circuit 8 and the BP circuit 10 to perform high-speed reconstruction processing.
In such a conventional apparatus, data associated with 360.degree. or 180.degree. views is basically processed as a group (unit). Image reconstruction is independently performed at each slice position by using a group of 360.degree. or 180.degree. views. For this reason, when images at a large number of slice positions are to be reconstructed, the time required for reconstruction is inevitably prolonged in proportion to the number of images. In addition, if reconstructed images are to be stored, the method of reconstructing images at a large number of slice positions one by one requires a large storage area.
Under the circumstances, methods of efficiently reconstructing images at many consecutive slice positions have been considered. For example, a reconstruction method is disclosed in U.S. Pat. No. 4,495,645.
This conventional continuous slice reconstruction method, however, is associated with a continuous scan scheme at the same slice position. No conventional methods have been proposed to allow efficient reconstruction of images at consecutive slice positions in the helical scan scheme.